Studies on aromatic compounds: Inhibition of calpain I by biphenyl derivatives and peptide-biphenyl hybrids

Article abstract of DOI:10.1016/j.bmcl.2004.03.071

With the objective to understand structural features responsible for the biological activity, novel nonelectrophilic biphenyl derivatives and peptide-biphenyl hybrids have been synthesized and evaluated as calpain I inhibitors. The preliminary results ind

Full text of DOI:10.1016/j.bmcl.2004.03.071

Bioorganic & Medicinal Chemistry Letters 14 (2004) 2753–2757
Studies on aromatic compounds: inhibition of calpain I
by biphenyl derivatives and peptide-biphenyl hybridsq
Ana Montero,a Mercedes Alonso,a Esperanza Benito,a Antonio Chana,a Enrique Mann,a
b
Jose M. Navas and Bernardo Herradon
a,
*
ꢀ
ꢀ
a
ꢀ
ꢀ
Instituto de Quımica Organica General, CSIC, Juan de la Cierva 3, 28006 Madrid, Spain
bDepartamento de Medio Ambiente, INIA, Ctra de la Coru~na Km 7.5, 28040 Madrid, Spain
Received 10 February 2004; revised 18 March 2004; accepted 25 March 2004
Dedicated to the memory of our colleague Dr. J. C. del Amo and to all the other victims of the terrorist attack. (Madrid, 11th March 2004)
Abstract—With the objective to understand structural features responsible for the biological activity, novel nonelectrophilic
biphenyl derivatives and peptide-biphenyl hybrids have been synthesized and evaluated as calpain I inhibitors. The preliminary
results indicate that the presence of additional aromatic rings (besides the biphenyl system) makes these compounds potent calpain
inhibitors with IC50 values in the nanomolar range.
Ó 2004 Elsevier Ltd. All rights reserved.
Calpain I (l-calpain) is an ubiquitous enzyme of the
calpain family of cysteine proteases that is activated by
micromolar amounts of Ca2þ ion.1;2 Although its natu-
ral substrate is not yet known, this enzyme has been
shown to hydrolyze a variety of proteins including
cytoskeletal proteins, enzymes involved in signal trans-
duction, membrane receptors and transcription factors.3
The overactivation of calpain is implicated in a variety
of pathophysiological conditions, including Alzheimer’s
disease, muscular dystrophy, multiple sclerosis, arthritis,
cataract and other degenerative and ageing-related dis-
eases.4
Accordingly, aldehydes,7 1,2-dicarbonyl compounds,8
a-hetero-substituted ketones9 and a,b-unsaturated car-
bonyl compounds,10;11 frequently incorporated in short
peptide chains, have been reported as calpain inhibitors.
However this strategy has resulted in poor selectivity:
many of the reported calpain inactivators are also
inhibitors of other proteases. On the other hand, the
distinctive mechanistic feature of calpain (i.e., activation
by Ca2þ) has not been extensively used as a basis for the
design.12 On analyzing the structure of calpain inhibi-
tors, it is observed the presence of aromatic rings (both
in the main peptide chain and as substituents) in many
synthetic inhibitors. Since aromatic compounds are
known to interact with a variety of chemical species,13
including Ca2þ ion,14 we can use this structural feature
for the design of nonelectrophilic calpain inhibitors.15
Although calpain I is a potential target for therapy, the
field is underdeveloped; the reported calpain inhibitors
are still few as compared to other proteases.5 Most of
the reported examples are based on similar strategies as
in the design of other protease inhibitors,6 that is to
exploit electrophilic functionality able to react with the
essential thiol group at the active centre of calpain.
Our group has investigated aromatic compounds,16
peptide derivatives11;17–19 and calpain inhibitors11;19 for
several years. During the course of these three interests,
we have recently synthesized compounds of type A (Fig.
1), termed peptide-biphenyl hybrids, where peptide chains
are linked to the positions 2- and 20- of the biphenyl
system.18 Our initial screening has shown that some
peptide-biphenyl hybrids A are able to inhibit calpain I.
The preliminary results indicate that the hybrids derived
from aromatic amino acids, such as 1–3, are potent and
selective calpain I inhibitors, having IC50 values in the
nanomolar scale.19 This activity may arise from the
Keywords: Calpain inhibitor; Biphenyl; Peptide-biphenyl hybrids;
Aromatic compounds.
q (a) Taken in part from the Ph. D. thesis of AC (Complutense
University, Madrid, 2003) and the projected Ph. D. thesis of AM and
MA. (b) A Spanish patent application (# 200301125, 14th May 2003)
is pending.
* Corresponding author. Tel.: +34-91-5618806; fax: +34-91-5644853;
e-mail: herradon@iqog.csic.es
0960-894X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved.
doi:10.1016/j.bmcl.2004.03.071

2754
A. Montero et al. / Bioorg. Med. Chem. Lett. 14 (2004) 2753–2757
H
N
CO2R
Ar
NH peptide
Ar
O
O
O
O
RO2C
N
H
peptide HN
1, Ar = Ph, R = CH3, IC50 = 64 nM
2, Ar = Ph, R = H, IC50 = 70 nM
3, Ar = 1H-Indol-3-yl, R = CH3, IC50 = 37 nM
A
Figure 1. Generic structure (A) of peptide-biphenyl hybrids and representative compounds (1–3) along with their IC50 values as calpain inhibitors.
capacity of compounds 1–3 to interact with Ca2þ ion,19c
hampering the activation of calpain. On continuing this
research, we report herein further examples on the
inhibition of calpain I by the peptide-biphenyl hybrids
(4–18, Fig. 2) that shed light on the influence of the
amino acid/peptide structures on the biological activity.
The synthesis of compounds 4–18 (Fig. 2) depends on if
the two chains at the positions 2- and 20- are identical or
not. If the molecule possesses C2-symmetry, the peptide-
biphenyl hybrid was prepared by the reaction of
biphenyl-2,20-dicarboxylic acid (19) with excess of the
corresponding N-unprotected amino acid or peptide
H
H
N
H
N
N
CO2CH3
CO2CH3
CO2R
Ar
Ar
R
O
O
O
O
R
O
O
CH3O2C
N
H
HO
RO2C
N
H
10
4, R = CH3
7, Ar = 1H-imidazol-4-yl, R = CH3
8, Ar = 4-OH-C6H4, R = CH3
9, Ar = 4-OH-C6H4, R = H
5, R = CH(CH3)2
6, R = CH(CH3)CH2CH3
O
H
N
H
Fe
N
CO2CH3
O
N
H
O
H
H
N
N
O
CH3O2C
S
N
O
Fe
N
H
H
O
O
OH
11
12
O
H
O
H
H
N
N
N
O
N
CO2CH3
O
N
H
H
N
H
H
CH3O2C
O
CH3O2C
N
O
O
N
H
N
H
O
S
O
S
14
13
O
O
H
N
H
N
O
N
CO2CH3
O
N
H
CO2CH3
H
H
N
H
CH3O2C
N
O
H3CO2C
O
N
N
H
S
H
O
O
16
15
OH
N
H
H
N
O
O
O
H
N
H
N
Fmoc
O
O
N
H
CO2CH3
O
N
H
CO2CH3
O
N
H
H
N
O
N
O
H
H
N
H
O
N
N
H
O
17
O
18
Figure 2. Structure of peptide-biphenyl hybrids essayed as calpain I inhibitors (see Table 1).

A. Montero et al. / Bioorg. Med. Chem. Lett. 14 (2004) 2753–2757
2755
Table 1. Results of the inhibition of calpain I by peptide-biphenyl
hybrids 4–18 and the peptide aldehyde 21
R-NH2 (2.6 mol equiv)
HOBt (2.6 mol equiv.)
EDC (2.6 mol equiv)
OH
NHR
O
O
Compound
IC50
O
O
Et3N (3.0 mol equiv)
DMAP (0.2 mol equiv)
DMF, rt
HO
RHN
4
Noninhibitor
7 lM
Noninhibitor
Noninhibitor
Noninhibitor
200 lM
5
6
55-89%
19
Scheme 1.
4-9, 11, 13, 16
7
8
9
10
11
12
13
14
15
16
17
18
21
Noninhibitor
24 lM
Weak inhibitor
98 nM
derivative according to standard peptide coupling
methodology (EDC/HOBt/DMAP; Scheme 1). Follow-
ing this method, the peptide-biphenyl hybrids 4–8, 11,
13 and 17 were synthesized in 79–94% yields.20;21 The
symmetric diacid 9 was prepared in quantitative yield by
hydrolysis (LiOH/H2O/THF) of the diester 8.
Weak inhibitor
77 lM
24 nM
41 lM
116 lM
240nM
If the substituents at positions 2- and 20- of the peptide-
biphenyl hybrid are different, the synthesis was carried
out sequentially from the anhydride 20 (Scheme 2). First
reaction with a slight excess of an N-unprotected amino
acid/peptide to give mono-amides of type B (such as 10,
Fig. 2) and a second reaction with another N-unpro-
tected amino acid/peptide in the presence of EDC/
HOBt/DMAP. Following this method, the hybrids 12,
14, 15, 17 and 18 were prepared in higher than 70%
overall yield.21
the compounds having IC50 values lower than
240nM are very active inhibitors.
(b) Most of the peptide-biphenyl hybrids with aliphatic
amino acids do not inhibit calpain, as shown by the
alanine (4) and isoleucine (6) derivatives. Other pep-
tide-biphenyl hybrids with longer aliphatic peptide
sequences at 2- and 20- of the biphenyl system (i.e.,
Ala-Leu–OMe,
D-Ala-D-Leu–OMe and Val-Ala-
Asp(OMe)2 derivatives, structures not shown) were
not active as calpain inhibitors.24 However, the
valine derivative methyl ester 5 is a moderate inhib-
itor of calpain I.
Compounds 4–18 were tested as calpain inhibitors using
a standard spectrofluorimetric method22 with labelled
casein as substrate and calpain I from porcine erythro-
cytes as enzyme. The results are indicated in Table 1.
The peptide aldehyde Z-Leu-Phe–H (21), that is one of
the most potent calpain inhibitors reported up to date
(Ki in the order of 10nM for human calpain, using
fluorescent N-succinyl-dipeptides as substrates),7;23 was
also tested under the same experimental conditions. This
result allows the direct comparison of the inhibitory
capacity of the peptide-biphenyl hybrids with reported
calpain inhibitors.
(c) The comparison of the biological activities of com-
pounds 1, 3, 7,25 and 8 show the striking dependence
of the biological activity on the nature of the aro-
matic amino acid: the derivatives of phenylalanine
1 and of tryptophan 3 are very potent calpain I
inhibitors (Fig. 1), while the analogues derived from
histidine (7) and tyrosine (8) are inactive. The diacid
9 derived from tyrosine is a weak inhibitor.
(d) Compounds 10–18 are biphenyl derivatives with at
least one phenylalanine residue, which can be con-
sidered as structural variants from the parent com-
pound 1. The acid 10 (inactive) is an analogue of 1
with a truncated chain and the diamide 11 (moder-
ate activity), derived from ferrocene,26 is a C-end
modified derivative of 1 and 2. The peptide-biphenyl
hybrids 12–17 are analogues of 1 with extended pep-
tide chains,27 and they show the influence of the
position of the phenylalanine residue, the nature of
the other amino acids and the length of the peptide
chain on the activity. It is found that the hybrids 12
and 14 are weak inhibitors,28 and compounds 15
The results indicated in Table 1 permit to draw some
conclusions:
(a) In our experiments, the known peptide aldehyde 21
has an IC50 value of 240nM. The discrepancy with
the reported result (Ki ꢀ 10nM) is not surprising
since two different parameters and two different sub-
strates are used to gauge the biological activity;
additionally, the sources of the enzyme are different
in the reported examples (human) and in the current
paper (porcine). Given that 21 is recognized as a
very potent calpain inhibitor, we can conclude that
R2-NH2 (1.3 mol equiv)
HOBt (1.3 mol equiv.)
R1-NH2 (1.1 mol equiv)
2,4,6-collidine
O
O
O
NHR1 EDC (1.3 mol equiv)
NHR1
(1.1 mol equiv.)
O
O
O
O
Et3N (1.5 mol equiv)
DMAP (0.1 mol equiv)
DMF, rt
DMF, rt
87-99%
R2HN
HO
79-94%
20
B (10)
12, 14, 15, 17, 18
Scheme 2.

2756
A. Montero et al. / Bioorg. Med. Chem. Lett. 14 (2004) 2753–2757
2. The two most abundant enzymes of the calpain-family are
and 17 are moderate inhibitors. On the other hand,
the hybrids 13 and 16, which have phenylalanine
residues at each second position of the peptide
chains, are excellent inhibitors with IC50 value in
the nanomolar range. The compound 18, which is
a structurally more elaborated peptide-biphenyl
hybrid having the aromatic amino acids separated
by the biphenyl residue and a densely functionalized
3,6-dihydro-2H-pyran,29 is a moderate inhibitor.
calpain I and calpain II (m-calpain), which are structurally
related. A significant difference is that calpain II requires
milimolar amounts of Ca2þ for activation (in vitro). The
rest of the enzymes of the calpain family are much less
abundant and most of them are tissue specific; for an
overview, see: Huang, Y.; Wang, K. K. W. Trends Mol.
Med. 2001, 7, 355.
3. (a) Lee, M.-S.; Kwon, Y. T.; Li, M.; Peng, J.; Friedlander,
R. M.; Tsai, L.-H. Nature 2000, 405, 360; (b) Bordone, L.;
Campbell, C. J. Biol. Chem. 2002, 277, 26673.
4. (a) Ray, S. K.; Hogan, E. L.; Banik, N. L. Brain Res. Rev.
2003, 42, 169; (b) Nixon, R. A. Ageing Res. Rev. 2003, 2,
407; (c) Biswas, S.; Harris, F.; Dennison, S.; Singh, J.;
Phoenix, D. A. Trends Mol. Med. 2004, 10, 78.
To conclude, the present paper reports further results on
qualitative structure–activity relationship of peptide-
biphenyl hybrids as calpain inhibitors. Although we
have not performed a systematic structural study,30 we
have found that the most potent inhibitors are those
having aromatic rings at the side chain, especially the
derivatives of phenylalanine. Less efficient calpain
inhibitors are the derivatives of tryptophan (i.e., 15),
whereas the derivatives of tyrosine and of histidine are
either weak or noninhibitors. In contrast, the peptide-
biphenyl hybrids derived from aliphatic amino acids are
not inhibitors,24 although the valine derivative 5 is an
exception.31 The combination of valine and phenylala-
nine residues in the peptide chains (i.e., the hybrid 16)
provides one of the most potent calpain inhibitors
reported up to date (10times more potent that the
peptide aldehyde 21). Whereas we can hypothesize that
the inhibition of calpain by compounds bearing several
aromatic rings can be due to its ability to coordinate
5. (a) Wang, K. K. W.; Yuen, P. Adv. Pharmacol. 1996, 37,
117; (b) Donkor, I. O. Curr. Med. Chem. 2000, 7, 1171.
6. (a) Otto, H.-H.; Schirmeister, T. Chem. Rev. 1997, 97, 133;
(b) Leung, D.; Abbenante, G.; Fairlie, D. P. J. Med.
Chem. 2000, 43, 305; (c) Powers, J. C.; Asgian, J. L.; Ekici,
€
O. D.; James, K. E. Chem. Rev. 2002, 102, 4639.
7. Chatterjee, S.; Iqbal, M.; Kauer, J. C.; Mallamo, J. P.;
Senadhi, S.; Mallya, S.; Bozyczko-Coyne, D.; Siman, R.
Bioorg. Med. Chem. Lett. 1996, 6, 1619.
8. (a) Donkor, I. O.; Zheng, X.; Han, J.; Lacy, C.; Miller, D.
D. Bioorg. Med. Chem. Lett. 2001, 11, 1753; (b) Lubisch,
W.; Moller, A. Bioorg. Med. Chem. Lett. 2002, 12, 1335;
(c) Bihovsky, R.; Tao, M.; Mallamo, J. P.; Wells, G. J.
Bioorg. Med. Chem. Lett. 2004, 14, 1035.
9. Tripathy, R.; Ator, M. A.; Mallamo, J. P. Bioorg. Med.
Chem. Lett. 2000, 10, 2315.
10. Ando, R.; Sakaki, T.; Morinaka, Y.; Takahashi, C.;
Tamao, Y.; Yoshii, N.; Katayama, S.; Saito, K.; Toku-
yama, H.; Isaka, M.; Nakamura, E. Bioorg. Med. Chem.
1999, 7, 571.
Ca2þ 19c
the present results show that this is not the only
,
cause, and a process of recognition of the inhibitor by
the enzyme is also operating (as observed from the
dependence of the biological activity on the sequence).
Finally, it is worthwhile to remark that these com-
pounds are some of the few calpain inhibitors without a
highly-reactive electrophilic functionality, what might
provide selectivity versus other proteases. The present
results also illustrate the use of the biphenyl fragment as
a privileged structure for the discovery of pharmaceu-
ticals.32 Work is in progress to further develop peptide-
biphenyl hybrids as calpain inhibitors as well as to fully
understand the relationship between structure and
activity.30
ꢀ
11. Mann, E.; Chana, A.; Sanchez-Sancho, F.; Puerta, C.;
ꢀ
ꢀ
Garcıa-Merino, A.; Herradon, B. Adv. Synth. Catal. 2002,
344, 855.
12. For studies on the structure and the mechanism of
activation, see: (a) Moldoveanu, T.; Hosfield, C. M.;
Lim, D.; Elce, J. S.; Jia, Z.; Davies, P. L. Nature Struct.
Biol. 2003, 10, 271, and references cited therein; (b)
PD150606, a derivative of a-mercaptoacrylic acid, is an
uncompetitive inhibitor of calpain that interacts with the
calcium-binding domains of both subunits of calpain; see:
Todd, B.; Moore, D.; Deivanayagam, C. C. S.; Lin, G.;
Chattopadhyay, D.; Maki, M.; Wang, K. K. W.; Nara-
yana, S. V. L. J. Mol. Biol. 2003, 328, 131, and references
cited therein; For other biological activities of PD150606,
see: (c) Van der Bosch, L.; Van Damme, P.; Vleminckx,
V.; Van Houtte, E.; Lemmens, G.; Missiaen, L.; Callewa-
ert, G.; Robberecht, W. Neuropharmacology 2002, 42, 706.
13. Hunter, C. A.; Lawson, K. R.; Perkins, J.; Urch, C. J.
J. Chem. Soc., Perkin Trans. 2 2001, 651.
Acknowledgements
Financial support from the Spanish Ministry of Science
and Technology (project
14. Ma, J. C.; Dougherty, D. A. Chem. Rev. 1997, 97, 1303.
15. Only one nonelectrophilic calpain inhibitor has been
reported, see: Wan, W.; DePetrillo, P. B. Biochem.
Pharmacol. 2002, 63, 1481.
ꢀ
16. (a) Herradon, B.; Cueto, S.; Morcuende, A.; Valverde, S.
Tetrahedron: Asymmetry 1993, 4, 845; (b) Heaton, N. J.;
#
BQU2001-2270) and
Fundaciꢀon‘la Caixa’ (project # 02/162-02) is acknowl-
edged. M.A. thanks CSIC for an I3P fellowship. J.M.N.
holds a Ramꢀon y Cajal contract from the Spanish Min-
istry of Science and Technology.
ꢀ
Bello, P.; Herradon, B.; del Campo, A.; Jimenez-Barbero,
J. J. Am. Chem. Soc. 1998, 120, 12371; (c) Mann, E.;
ꢀ
ꢀ
ꢀ
Mahıa, J.; Maestro, M. A.; Herradon, B. J. Mol. Struct.
2002, 641, 101; (d) Chana, A.; Concejero, M. A.; de
References and notes
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Frutos, M.; Gonzalez, M. J.; Herradon, B. Chem. Res.
Toxicol. 2002, 15, 1514; (e) Bello, P.; Heaton, N. J.;
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1. (a) Sorimachi, H.; Ishiura, S.; Suzuki, K. Biochem. J. 1997,
328, 721; (b) Goll, D. E.; Thompson, V. F.; Li, H.; Wei,
W.; Cong, J. Physiol. Rev. 2003, 83, 731.
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Chana, A.; Jimenez-Barbero, J.; Riande, E.; Herradon, B.
J. Phys. Org. Chem. 2004, 17, 71.
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A. Montero et al. / Bioorg. Med. Chem. Lett. 14 (2004) 2753–2757
2757
ꢀ ꢀ
17. Salgado, A.; Mann, E.; Sanchez-Sancho, F.; Herradon, B.
Heterocycles 2003, 60, 57.
18. Mann, E.; Montero, A.; Maestro, M. A.; Herradon, B.
pressure; the residue was taken up with EtOAc and
washed with 5% aqueous HCl, water, saturated aqueous
NaHCO3 and brine. The organic phase was dried
(MgSO4). After filtration and solvent removal, the residue
was purified by chromatography (1:1 to 1:4 gradient of
hexane/EtOAc) to give 12 as a white solid (1.28 g, 94%
yield).
ꢀ
Helv. Chim. Acta 2002, 85, 3624.
19. (a) Montero, A.; Mann, E.; Chana, A.; Herradon, B.
ꢀ
Chem. Biodiv. 2004, 1, 442; (b) Compounds 1–3 do not
inhibit trypsin and papain; (c) The interaction of peptide-
biphenyl hybrids with Ca2þ and other cations have been
proved experimentally (1H NMR and MS) and computa-
tionally;19a a comprehensive study is underway.
22. (a) Jiang, S.-T.; Wang, J.-H.; Chang, T.; Chen, C.-S. Anal.
Biochem. 1997, 244, 233; (b) Jones, L. J.; Upson, R. H.;
Haugland, R. P.; Panchuk-Voloshina, N.; Zhou, M.;
Haugland, R. P. Anal. Biochem. 1997, 251, 144; (c)
20. (a) Since that the purpose of our research was to
investigate the properties of the target molecules we have
not made any attempt to optimize the synthetic proce-
dures. However, with the aim to gain some insight on the
synthetic methodology for these compounds, we have
prepared some peptide-biphenyl hybrids by more than one
method. Although the more frequent reactions are those
indicated in Schemes 1 and 2, other methods include: (a)
The reaction of 1,10-biphenyl-2,20-dicarbonyl dichloride
with 2.4 molar equiv of N-unprotected amino acid/peptide
in the presence of Et3N (60–90% yield)18; (b) The
phenylalanine derivatives 11 and 13 were also synthesized
from the acid 2 and the corresponding amine (HOBt/
~
Thompson, V. F.; Saldana, S.; Cong, J.; Goll, D. E. Anal.
Biochem. 2000, 279, 170.
23. Mehdi, S.; Angelastro, M. R.; Wiseman, J. S.; Bey, P.
Biochem. Biophys. Res. Commun. 1988, 157, 1117.
24. Other peptide-biphenyl hybrids having only aliphatic
amino acids were also tested for calpain inhibition, finding
that 5 is the sole calpain inhibitor in this family of peptide-
biphenyl hybrids (Chana, A. Ph. D. thesis, Complutense
University, Madrid, 2003).
25. Compound 7 was used as an intermediate for the synthesis
of imidazo-copper complexes, see: Amine, A.; Atmani, Z.;
ꢀ
El Hallaoui, A. A.; Giorgi, M.; Pierrot, M.; Reglier, M.
Biorg. Med. Chem. Lett. 2002, 12, 57.
ꢀ
EDC/DMAP) in over 90% yield; (c) Najera’s methods
ꢀ
ꢀ
(HOTT/DMF; see: Najera, C.; Bailen, M. A.; Chinchilla,
R.; Dodsworth, D. J. Org. Chem. 1999, 64, 8936) was
quite suitable for the synthesis of peptide-biphenyl hybrids
having valine at the N-end of the peptide chain (70–90%
26. Peptides with ferrocenyl fragments can be useful in
asymmetric catalysis as well as in the design of techno-
logical useful materials; see: (a) Dai, L.-X.; Tu, T.; You,
S.-L.; Deng, W.-P.; Hou, X.-L. Acc. Chem. Res. 2003, 36,
659; (b) Pal, S. K.; Krishnan, A.; Das, P. K.; Samuelson,
A. G. J. Organomet. Chem. 2001, 637–639, 827.
27. Some peptide-biphenyl hybrids with chains of three
aromatic amino acids were also tested, but they were not
active. In any case, the longer peptide-biphenyls are
expected to have limited therapeutic value, since their
poor bioavailability (low transport across cell membrane
and easier hydrolysis by proteases).
28. Weak inhibition means that the compound inhibited
calpain I in the preliminary screening, but its IC50 was
higher than 200 lM.
29. (a) Peptides on functionalized 3,6-dihydro-2H-pyran
template were prepared from 2-hexenopyranosides
using a Claisen rearrangement as the key step: (a)
Mann, E. Ph. D. thesis, Autonoma University, Madrid,
ꢀ
yield); we thank Professor Najera for a generous gift of
S-(1-oxido-2-pyridinyl)-1,1,3,3-teramethyluronium hexa-
fluorophosphate (HOTT).
21. All the new compounds gave satisfactory analytical (C, H,
N, 0.3%) and spectroscopic (1H NMR, 13C NMR, IR,
MS) data. (b) The synthesis of compound 12 illustrates the
two main procedures used for the synthesis of the peptide-
biphenyl hybrids. The experimental details are as follows.
A solution of Tyr–OMe (1.13 g, 4.90mmol) and 2,4,6-
collidine (0.65 mL, 4.90 mmol) in dry DMF (12.5 mL) were
dropwise added to a solution of 20 (1.0g, 4.46 mmol) in
dry DMF (12.5 mL) under argon atmosphere. The mixture
was stirred at room temperature for 15 h. Then, the
solvent was removed under reduced pressure, the residue
was taken up with EtOAc and washed with 5% aqueous
HCl (twice). The organic phase was dried (MgSO4). After
filtration and solvent removal, the residue was purified by
crystallization from CH2Cl2 to give pure acid (1.8 g, 97 %
yield.) of type B (with R1 corresponding to Tyr–OMe). A
mixture of the intermediate acid (800 mg, 1.91 mmol) and
the trifluoroacetate salt of Phe-Cys–OMe (1.05 g,
1.91 mmol) in dry DMF (17 mL) was sequentially treated
with HOBt (335 mg, 2.48 mmol), Et3N (0.40 mL,
2.85 mmol), EDC (495 mg, 2.48 mmol) and DMAP
(24 mg, 0.16 mmol) at room temperature under argon
atmosphere. The mixture was stirred at room temperature
for 32 h. Then, the solvent was removed under reduced
ꢀ
2002; (b) Montero, A.; Mann, E.; Herradon, B. Eur. J.
Org. Chem., in press.
30. Work is in progress to prepare combinatorial libraries of
peptide-biaryl hybrids with systematic substitution of the
amino acids (Montero, A.; Albericio, F.; Royo, M.;
ꢀ
Herradon, B. Work in progress).
31. We do not have an explanation for this behaviour,
although we can speculate that the steric requirements of
valine residues influence the conformation of peptide-
biphenyl hybrids.
32. Hajduk, P. J.; Bures, M.; Praestgaard, J.; Fesik, S. W.
J. Med. Chem. 2000, 43, 3443.